The invention relates to a device for continuously detecting pinholes or tears in a foil during deformation of the foil, having a device for clamping the foil, a device for controlled deformation of the foil, at least one light source situated on one side of the foil, at least one light sensor on the other side of the foil for measuring the light penetrating the foil, and a device for continuously registering the light intensity measured by the light sensors.
In order to test foil-shaped materials for their tendency to form pin-holes or tears during shape-forming, the multi-axial stretch (MAS) test for example is employed. This method of testing is a stretch-draw test using a 27 mm diameter cylindrical Teflon punch (also called male forming die) and 30 mm diameter die. By plotting the force distance relationship during the deformation the depth at which fracture occurs is obtained as an expression of the formability of the foil being tested. In special cases, in addition to this test, the depth up to which the foil is free of pin-holes is determined. To that end, cups of increasing depth are prepared using the device employed for the MAS-test, and the first pin-holes subsequently detected by inspecting the cups visually using a light source. By investigating the deformation behaviour of various materials it has been found that there is not always direct correlation between depth at failure and depth at which the material is free of pin-holes.
It is also known to have a light-permeability pin-hole detector situated after the shape-forming station of production units manufacturing blister packs and other forms of foil packaging with recesses shape-formed in the foil.
State-of-the-art technology includes a device of the kind described at the start illustrated in FIG. 2 in which the device for controlled deformation of the foil comprises a compressed gas device for applying to one side of the foil increasing gas pressure and a device to measure the degree of deformation continuously. With this device it is possible to detect pin-holes or tears directly as they are formed during the shape-forming of the foil. One significant disadvantage of this pin-hole detector is, however, that, because of the absence of friction during shape-forming with compressed gas, the flow behavior of the foil during deformation differs greatly from the actual flow behavior of foil deformed in practice using a shape-forming punch. For that reason, the knowledge of pin-hole formation and tearing gained from the compressed gas method can be applied only to a limited degree to the normal practice of shape-forming with a punch.
The object of the present invention is therefore to provide a device of the kind described at the start, which does not exhibit the disadvantages of the known pin-hole detectors and which leads to a deformation behaviour which corresponds to a large degree to the forming behaviour in practice.
That objective is achieved by way of the invention in that the device for controlled deformation of the foil comprises a punch which is situated on one side of the foil and is connected to a drive mechanism for deforming the foil, the light source or light sensors being integrated in the punch.
With the arrangement according to the invention in which the light source or the light sensors is/are situated within the punch, it is possible to copy the deformation with a shape-forming punch performed in practice.
The device according to the invention offers considerable advantages over the state of the art devices viz.:
using on-line testing for the light permeability of the foil, it is possible to detect the first pin-holes or tears and to observe the corresponding behaviour of the foil during sub-sequent deformation, this in a simple manner which is close to conditions in practice without visual testing of a series of samples in order to determine the pin-hole free depth in cups deformed to different depths.
the pin-holes are detected immediately as they form i.e. at a point in time in which the foil being subjected to deformation is under tensile stress. With the normal means of measurement the pin-hole free depth it is not possible to detect pin-holes so early as, under the absence of tensile force, these become so much smaller that they are no longer detectable visually. The same applies for a pin-hole detector situated after a shape-forming station.
A preferred version of the device according to the invention is such that at least shape-endowing region of the punch is of an easy-slide, light-permeable material, in particular of Teflon, whereby the light permeability of the punch surface may be increased further by use of a porous material or by micro-perforation.
The easy-slide material may be deposited on the shaping punch as a coating. In a particularly useful version the punch is, however, in the form of a thin-walled hollow body with light-permeable wall.
In a particularly preferred version of the device according to the invention the shaping punch faces a die serving as a black chamber and the light sensors are situated in the die.
The device according to the invention may be employed not only for material testing purposes but is also suitable for use in industrial production units employed for the production of foils with recesses, for example blister packs.
The device according to the invention can supplement or replace the present day pin-hole detector normally installed after the shape-forming station. As the shaping punch fitted with a light source can be employed to manufacture recesses e.g. in blister manufacture andxe2x80x94along with light sensors in the die of a shaping stationxe2x80x94as a pin-hole detector, it is possible to obtain optimal control of the deformation process.
A special area for using the process or device according to the invention is in the testing of foils that are non-permeable to light, in particular foil laminates of plastic and metal or metal foils, preferably foil laminates part of which is of aluminium or aluminium foils.